Reaction condition: 10 g
solvent,
1.0 wt. % lignin, 0.1 mmol CuSO4, 1.0 MPa
O2, 433 K, 4 h. Note: H, G and S represent the products
from the structural units of the p -coumaryl alcohol (H),
coniferyl alcohol (G), and sinapyl alcohol (S) in lignin respectively.
a-without CuSO4.
Additionally, the structures of raw lignin and Re-lignins from different
oxidation system were analyzed by FT-IR spectra (Figure 6). The
absorbances at 3450 and 2912 cm-1 can be assigned to
the vibration of O-H and C-H stretching respectively, and those at 1605,
1514, 1454 and 814 cm-1 are ascribed to the aromatic
skeletal vibrations of lignin,42 which suggests that
the skeletal structure of lignin is retained in Re-lignins. It is worth
pointing out that the absorbance at 1740 and 1103 cm-1(characteristic absorption of C=O stretching and the H structural unit
in lignin respectively) for Re-lignins from the microemulsion (point c
in Figure 3) are weakened more significantly by comparison to that from
other system, this observation further certifies that the developed
microemulsion reactor is selective to tailor H unit in lignin.